XA9847763 -
IAEA-TECDOC-1028
Use of 137
soil erosion and sedimentation
Proceedings of a consultants meeting organized by the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture and held in Vienna, 13-16 November 1995
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USE OF 137Cs IN THE STUDY OF SOIL EROSION AND SEDIMENTATION
IAEA, VIENNA, 1998 IAEA-TECDOC-1028
ISSN 1011-4289
©IAEA, 1998
Printed by the IAEA in Austria July 1998
FOREWORD
Soil erosion and sediment deposition represent serious threats worldwide because of their impact on agricultural production and environmental conservation. Erosion affects the productivity of soil through loss of the nutrient-rich surface layers and the incorporation of potentially growth- limiting subsoil into the rooting zone. In many cases, erosion causes progressive decline in soil productivity, particularly so in agro-ecosystems that rely on indigenous fertility. The use of high- input technology such as large amounts of fertilizers, pesticides, and irrigation helps offset deleterious effects of erosion but has the potential to create pollution and health problems, destroy natural ecosystems, and contribute to high energy consumption and unsustainable agricultural systems.
Soil erosion causes not only on-site degradation of a precious natural resource, but also off-site problems of sediment deposition in residential areas and reservoirs, eutrophication of surface waters and pollution from various particle-adsorbed toxic agrochemicals.
Erosion and deposition are recognized to have occurred throughout the history of agriculture, and notwithstanding a half-century of research into its causes and effects, considerable uncertainty persists about extent, magnitude and actual rates, as well as on the economic and environmental consequences. When economic costs of soil loss and degradation and off-site effects are conservatively estimated into cost/benefit analyses of agriculture, it makes sound economic sense to invest in programs that are effective in the control of soil erosion.
The use of radionuclides in soil erosion/deposition research overcomes many of the problems associated with traditional approaches and is now being applied successfully in several developed countries. Among these, the
I37Cs technique allows the assessment of both soil loss and deposition in the same watershed from a single site visit without the need for long-term financial commitments.
Caesium-137, an artificial radionuclide with a half-life of 30.2 years, is distributed across the earth's surface due to fallout from atmospheric nuclear tests and accidental releases from nuclear reactors.
Strongly adsorbed by clay particles, it provides a unique tracer of soil movement.
In response to the United Nations Conference on Environment and Development convened in Rio de Janeiro in June 1992, the UN system launched a worldwide environmental programme called EARTHWATCH. The IAEA joined this initiative through a series of activities on environmental monitoring, impact assessment and environmental protection. Thus, in connection with Chapter 12,
"Managing Fragile Ecosystems: Combating Desertification and Drought", of Agenda 21, two IAEA
Divisions, the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture and the
Division of Physical and Chemical Sciences, joined forces to plan, organize and implement activities
on the assessment of soil erosion and sedimentation as a basis for soil conservation and environmental
protection. A panel of experts on the use of isotopes in studies on soil erosion convened in November
1995 in Vienna to discuss the possibilities of exploiting radionuclide methodologies, and
I37Cs in
particular, for the assessment of soil erosion and sedimentation in developing countries in which food
security is at greatest risk. The state-of-the-art reports presented at that meeting are contained in this
publication which, as the first comprehensive treatment of the subject, is expected to serve as an
invaluable source of information to underpin future research on soil conservation and environmental
protection. The responsible IAEA officer was F. Zapata. The assistance of D.J. Pennock and
A.R.J. Eaglesham in the preparation of this publication is gratefully acknowledged.
EDITORIAL NOTE
In preparing this publication for press, staff of the IAEA have made up the pages from the original manuscripts as submitted by the authors. The views expressed do not necessarily reflect those of the IAEA, the governments of the nominating Member States or the nominating organizations.
Throughout the text names of Member States are retained as they were when the text was compiled.
The use of particular designations of countries or territories does not imply any judgement by the publisher, the IAEA, as to the legal status of such countries or territories, of their authorities and institutions or of the delimitation of their boundaries.
The mention of names of specific companies or products (whether or not indicated as registered) does not imply any intention to infringe proprietary rights, nor should it be construed as an endorsement or recommendation on the part of the IAEA.
The authors are responsible for having obtained the necessary permission for the IAEA to
reproduce, translate or use material from sources already protected by copyrights.
CONTENTS
Summary . . . 1
137
Cs use in estimating soil erosion: 30 years of research .. . .. . . . . . . . 5 J.C. Ritchie
New perspectives on the soil erosion-soil quality relationship . . . 13 D.J. Pennock
Deposition, transfer and migration of
I37Cs and
90Sr in Swedish
agricultural environments, and use of
!37Cs for erosion studies . . . 27 E. Haak, T Rydberg
Use of
l37Cs and other fallout radionuchdes in soil erosion investigations:
Progress, problems and prospects . . . 39 D.E. Walling
Bibliography of publications of
137Cs studies related to soil erosion and
sediment deposition . . . . . . . . . . 63
J.C. Ritchie, CA RitchieGlobal distribution of
137Cs inputs for soil erosion and sedimentation studies . . . 117 E. Garcia Agudo
List of Participants . . . . . . 123
SUMMARY
Although current concern for the global environment focuses largely on problems of global warming and climate change, there is increasing realization that soil erosion represents a major constraint to sustamable development of agricultural production in a world characterized by a burgeoning population Soil erosion is a natural process caused by water and wind, but the accelerative effects of human activities, most notably deforestation and poor agricultural practices, are of increasing concern Soil erosion causes not only on-site effects associated with compromised soil productivity and loss of agricultural land, but there are also many off-site problems such as
downstream sediment deposition, pollution of water courses with various adsorbed agrochemicals andeutrophication of water bodies It is estimated that the world's arable lands are, on average, being eroded at a rate considerably in excess of that of soil formation, resulting in a net 7% depletion of soil resources each decade, with potentially serious implications for food security in many countries Furthermore, the world's reservoirs are losing storage capacity at about 50 km
1(approximately 1%) per year, as a result of sedimentation Considering the central importance of reservoirs for domestic and industrial water supply, for irrigation schemes, and for hydropower production, and thus for economic progress in many developing countries, this trend is of significant concern
To date, much of the loss of soil and agricultural land through erosion has been compensated by clearing new farm land, and by use of fertilizer and improved crop varieties to increase yields on existing land However, the scope for maintaining such compensatory measures will decline in the future
Against this background, there is an increasing need to assemble reliable information on rates of soil loss in different areas of the world Such information is needed to precisely assess the magnitude of the problem, to evaluate influencing factors, to validate existing and new prediction models, and to investigate relationships with crop production This need is, however, not readily met by classical methods of measurement of erosion, which have significant limitations They do not give unbiased measurements of soil redistribution, and, more importantly, they do not address spatial patterns of erosion and deposition within fields There is a need for location-specific measurements within the landscape, especially in areas where other erosion data are not available and where long- term experiments have not, nor cannot be, established
The use of radionuchdes overcomes many of the problems associated with the traditional approaches and they have the potential to provide the needed data By labelling the soil, both the extent and the source of soil loss can be determined Several radionuchdes, mainly gamma emitters have been applied as tracers in field-erosion studies (
59Fe,
46Sc, "°Ag,
:<>8Au,
n4Cs,
MCr) Another group includes the environmental radionuchdes, such as
n7Cs,
2:0Pb,
7Be,
240Pu,
I4C, ^
2Si,
26A1 and
15
C1, which can be used to assess soil erosion and deposition patterns and related problems depending on the time scale involved
The use of fallout radioactivity to estimate soil loss was first published in 1960, the transport of Sr in runoff from "standard" erosion plots showed that tracer depletion was greatest where the
onmost soil loss occurred Research in the late 1960s showed that differences in distribution of fallout
n7
Cs between vegetation types on a catchment in Mississippi were due to soil loss from the landscape In the mid-1970s, the combination of data from a variety of soil and erosion conditions, with fallout radionuchdes and added tracers, demonstrated a significant exponential relationship between soil loss and tracer loss, with the basic conclusion that
n7Cs would be a useful tool for
measuring erosionCaesium-137 is an artificial radionuchde with a half-life of 302 years produced by nuclear fission Widespread global distribution of l37Cs into the environment began with atmospheric testing of high-yield atomic weapons in the 1950s and the early 1960s To a lesser extent atmospheric explosions continued until 1980 The n?Cs and other radionuchdes were released into the
stratosphere, distributed globally, then moved back to the troposphere and to the earth's surface as fallout, the amount of deposition depending on the atmospheric concentration and rainfall.
Radioactive fallout has been monitored globally since the early 1950s. Strontium-90, identified as the most dangerous radionuclide in fallout due to its biological fixation in bones and the consequent link with bone cancer, was most closely followed, with a map of cumulative deposits published in 1967. Although '
37Cs deposition is less well documented, it is possible to construct its deposition pattern from the ^Sr data.
Caesium-137 is by far the most widely used fallout radionuclide in soil erosion and sedimentation research by virtue of its strong adsorption to fine soil particles, a relatively long half- life, ease of measurement and well defined patterns of fallout input. Thus, in agro-ecosystems, its redistribution is a direct indication of erosion, transport and deposition of soil particles occurring during the period extending from the main phase of atmospheric deposition to the time of sampling.
Assessment of
137Cs redistribution is commonly based upon comparison of measured inventories (total activity per unit area) at individual sampling points, with an equivalent estimate of the inventory representing the cumulative atmospheric fallout at the site, taking due account of the differing behaviour of cultivated and uncultivated soils. Because direct long-term measurements of atmospheric fallout are rarely available, the cumulative input or reference inventory is usually established by sampling an adjacent undisturbed, putatively uneroded location, generally under permanent pasture, which provides an estimate of total fallout. Where sample inventories are lower than the local
reference inventory, loss of caesium labelled soil and therefore erosion may be inferred. Similarly,sample inventories in excess of the reference level are indicative of addition of
:37Cs-labelIed soil by deposition. The magnitude and direction of the measured deviations from the local reference level provide a qualitative assessment of soil redistribution. In order to derive quantitative estimates of rates of erosion and deposition from
l37Cs measurements, it is necessary to establish a relationship between the magnitude of the deviation from the reference inventory and the extent of soil loss or gain.
Because empirical calibration data are rarely available, many workers have favoured the use of theoretical relationships or models to provide the necessary calibration function.
Although the basis for the use of the
137Cs technique to document rates and patterns of soil loss is attractive in its simplicity, it is founded on several key assumptions and a number of potential limitations and uncertainties must be recognized and addressed in any application. However, limitations inherent in the use of fallout
1T!Cs to estimate erosion are no greater than those associated with other techniques used by soil scientists and geomorphologists. If studies are designed to mitigate the effects of these limitations, then measuring the concentration of
l37Cs across the landscape can provide reliable and valuable information on erosion and deposition.
A significant expansion in the application of the l37Cs approach has occurred recently, encompassing various locations ranging from glaciated mountain areas in Greenland, through the
prairie and steppe regions of Canada and the Russian Federation and semi-arid areas of Spain, to tropical areas of Africa and Thailand. Therefore, the approach must now be seen as having global relevance.
It should be noted, however, that additional inputs of
137Cs fallout to many parts of Europe, associated with the Chernobyl accident in April 1996, have complicated the interpretation of the
l37Cs
inventories and may render the approach of limited value in such areas.Although the validity and value of the
137Cs approach has now been demonstrated by numerous
studies in a number of environments, considerable scope remains to develop, refine and standardize
the procedures employed and to explore additional applications. To date, most of the studies using
this approach have been concerned with soil erosion by water, but there is clear potential to extend its application to redistribution of soil by tillage and wind erosion.The data generated by
I37Cs measurements are ideally suited to coupling with GIS and spatial statistics, and for use in verifying computer-generated models for distributed erosion and soil loss Scope also exists for using estimates of soil loss derived from radiocaesium measurements as a basis for establishing medium-term soil-loss/crop-productivity relationships In addition, there is potential for extending the use of
I37Cs as a sediment tracer from consideration of soil redistribution within individual fields to investigation of the movement and storage of sediment within a drainage basin
Other fallout radionuchdes, including unsupported
2l°Pb and
7Be have attracted much less
attention, but there is increasing evidence that they offer considerable potential for use in soil-erosion research, individually and complementary to 137Cs The International Atomic Energy Agency, throughseveral mechanisms, is promoting further applications of fallout radionuchdes in soil erosion and
sedimentation studiesNEXT PAGE(S)
137
Cs USE IN ESTIMATING SOIL EROSION: 30 YEARS OF RESEARCH J.C. RITCHIE
Beltsville Agncultural Research Center, XA9847764 Beltsville, Maryland,
United States of America Abstract
137C.s USE IN ESTIMATING SOIL EROSION: 30 YEARS OF RESEARCH
Significant amounts of fallout l37Cs from nuclear weapons tests were introduced to the landscape during the 1950s and 1960s. Once 137Cs reaches the soil surface it is strongly and quickly adsorbed by clay particles, and is essentially nonexchangeable in most environments. Thus, 137Cs becomes an effective tracer of the movement of soil particles across the landscape. Over the past 30 years, researchers have shown that l37Cs can be used to study soil movement. Early work used empirical relationships between soil loss and 137Cs loss to estimate erosion. This was followed by the development of proportional and theoretical models to relate 137Cs movement and soil redistribution. Most of the problems related to the
137Cs technique are the same as those encountered with other techniques (i.e., sampling, measurement). The l37Cs technique can make actual measurements of soil loss and redeposition in fields, fostering the formulation of better plans to conserve the quality of the landscape. This paper reviews the development of the 137Cs technique to show how it can be used to understand erosion and soil movement on the landscape.
1. INTRODUCTION
Soil erosion is a natural process caused by water and wind. The accelerative effects of man's activities on erosion and the off-site damage caused, are major concerns around the world. The size of the problem and concern over degradation of the landscape are well documented [1, 2 ,3].
Economic effects of soil erosion along with off-site, downstream damage from eroded soil particles have also been described [3, 4, 5].
Measurements of soil erosion on the landscape using classical erosion techniques are difficult, time consuming, and expensive [6]. Empirical and theoretical mathematical equations/models have been developed. The most widely used is the Universal Soil Loss Equation (USLE), which is an empirical-based equation developed with data collected from soil erosion plots on "typical" soils of the United States east of the Rocky Mountains [7]. The USLE has been used and misused in the United States and around the World [8J. However, it is still the most widely used, powerful and practical tool for estimating sheet and rill erosion on the landscape. A Revised Universal Soil Loss Equation (RUSLE) is currently being used in the United States with applications to a wider range of conditions and locations than the original USLE [9]. There are many other efforts to model soil erosion and its off-site effects [10] that have had varying degrees of success and applications in management and research. One such effort is the Water Erosion Prediction Project (WEPP), which is a process-based, simulation model of soil erosion [11].
Over the past 30 years, researchers have studied the potential of using natural and man-made
radioisotopes to study the erosion and sediment-deposition cycle. Several radioisotopes have been
used. The potential for using fallout
137Cs to provide independent measurements of actual soil erosion
rates and patterns and sediment deposition is well documented [12, 13]. The purpose of this paper is
to review the development of the
137Cs technique for studying erosion and redeposition, based on a
bibliography [14] of 1500 papers that show extensive use of the technique globally.
2. BACKGROUND
Most classical methods for estimating soil erosion are based on measuring soil loss from plots or at the edge of a field. They do not give unbiased measurements of actual soil movement, and, more importantly, they do not address spatial patterns of erosion and redeposition within fields.
Mathematical models have the same limitations. There is a need to be capable of making measurements at any location on the landscape, especially in areas where other erosion data are not available and where long-term experiments have not, nor cannot be, established. Classical erosion measurement techniques and mathematical models cannot meet these criteria. Tracer techniques have the potential to provide the necessary type of data. However, such techniques can be difficult if tracer must be added to the environment. A tracer is needed that is naturally distributed across the landscape, easily measured, and readily adsorbed to soil particles.
Fallout
137Cs from atmospheric nuclear weapons tests of the 1950s and 1960s is a unique tracer for the erosion and deposition cycle because no natural sources are in the environment. Yet,
137Cs is globally distributed across the earth's surface due to fallout deposition from such tests and releases from nuclear reactors [15, 16]. Before 1952, releases were localized around weapon test sites or reactors. With the coming of high-yield thermonuclear weapons testing in November 1952 [17],
137Cs was injected into the stratosphere and circulated globally [18]. Fallout rates decreased with distance from the northern temperate zone. Regional patterns and rates of fallout were linearly related to precipitation in latitudinal zones [18].
Local variation of fallout
137Cs on the landscape can be significant. Studies have reinforced the need to make local measurements at undisturbed sites rather than by extrapolating from values determined at other locations [19, 20]. The rates of deposition of fallout
137Cs have decreased since the maxima of the early 1960s, and since the mid-1980s have often been below detection limits [15].
Releases from nuclear reactors are usually local in nature. However, the Chernobyl accident in April 1986 caused regional dispersal of measurable
I37Cs [21] that affected the total global deposition budget [22]. Thus man's activities related to nuclear energy have distributed a unique radioactive element across the landscape surface in discernible patterns that can be used to trace natural events.
The chemistry of this unique tracer is well understood [18, 23]. Once
137Cs reaches the soil surface it is strongly and quickly adsorbed by clay particles, and is essentially nonexchangeable in most environments [24, 25, 26]. Thus,
137Cs becomes an effective tracer of the movement of surface soil. Distribution of
137Cs in soil profiles at undisturbed sites shows an exponential decrease with depth [27, 28, 29], whereas plowed soils show uniform distribution throughout the plowed layer [30, 31].
Less than 1 % of the
137Cs is flushed in solution from a catchment immediately after deposition, and generally less than 0.1% moves in solution per year after the initial flush [32, 33]. Thus most movement of
137Cs across the landscape is due to the physical processes of erosion and sediment deposition.
Accurately measuring
137Cs in environmental samples is easy [34, 61]. In soil erosion studies, the challenge is to elucidate the changing patterns of distribution of
137Cs-tagged soil particles on the landscape. The redistribution of
137Cs between and within landscape elements provides information on soil erosion rates and patterns. Although biological and chemical processes move limited amounts of
137
Cs in unique environments, water and wind are the dominant factors moving
137Cs-tagged soil particles between and within compartments of the landscape.
Thus, measurement of
137Cs redistribution on the landscape provides estimates of long-term
soil loss. Estimates are location-specific and can be made with minimum disturbance to study sites,
giving both spatial patterns and rates of erosion from a single visit.
3. EARLY RESEARCH
The first publication of the use of fallout radioactivity to estimate soil loss was in 1960 by Menzel [35], who measured the transport of fallout ""Sr in runoff from "standard" erosion plots. He concluded that
wSr loss was greatest from those plots that had the most soil loss. Although this study did not include
137Cs, it showed that the movement of fallout radioactivity on the landscape was related to soil movement.
In 1963, Frere and Roberts [36] measured ""Sr across a small cultivated catchment in Ohio as a function of slope position and shape, and concluded that the pattern was due to the redistribution of soil particles by erosion processes. Graham [37] added
85Sr and
I31I to standard erosion plots, and concluded that soil particles affected nuclide removal by runoff water. And Yamagata, Matsuda, and Kodaira [38] concluded that runoff was a factor in the removal of
137Cs and *°Sr from catchments.
In 1965, Rogowski and Tamura published the first of three reports [39, 40, 41] on the movement of
137Cs by runoff, erosion, and filtration from plots at the Oak Ridge National Laboratory in Tennessee, USA. They added
137Cs as a tracer and followed its movement by measuring runoff, soil loss and
137Cs loss at a flume at the end of erosion plots. The first publication [39] was based on the first 83 days of data collection. In follow-up publications in 1970, they discussed the environmental mobility of
137Cs [40] and erosional behavior of
137Cs [41]. They found a significant exponential relationship between soil and
137Cs losses, and concluded that erosion was a major factor in removing
137Cs. Although these studies were not based on fallout nuclear weapons tests, they showed that
137Cs and soil movements were related, and could be used as a tool for estimating soil redistribution on the landscape.
It is interesting that in 1965 Wischmeier and Smith [7] first published the USLE (Universal Soil Loss Equation) in USD A Agriculture Handbook No. 262 on predicting rainfall-erosion losses for cropland east of the Rocky Mountains. The USLE, an empirical equation based on measured soil loss from standard erosion plots on "typical soils" east of the Rocky Mountains in the United States, received wide acceptance in the United States and around the world. The equation is used and misused [8] for areas where the empirical relationship developed for United States soils is probably not applicable. The early work of Rogowski and Tamura on using the
137Cs to measure soil loss, however, has yet to be implemented as a tool for soil conservationists to study soil redistribution.
In another study at Oak Ridge, Dahlman and Auerbach [42] added
137Cs to a grass plot (fescue meadow) and used its redistribution to estimate erosion. They also found an exponential relationship between soil and
137Cs losses. These early studies with high levels of added
137Cs raised the question of whether measurement of the much lower fallout levels could be used to estimate soil redistribution patterns across the landscape.
In 1968, Ritchie and McHenry began a series of studies to determine if fallout levels of
137Cs
could be used as a tracer of sediment movement and deposition across natural and agricultural
landscapes, supported jointly by the United States Department of Agriculture and the U.S. Atomic
Energy Agency (now Department of Energy). In 1970 [43, 44], they concluded that differences in
distribution of fallout
137Cs between vegetation types on a catchment in Mississippi were due to soil
loss from the landscape. In 1972 [45, 46, 47], they designed an experiment to determine the
relationship between losses of soil and of fallout
137Cs in a catchment. Soil loss was estimated using
the USLE and fallout
137Cs loss was calculated as a percent compared with a non-eroded reference
site. They found an exponential relationship between soil and
137Cs loss, and, along with data from
two other catchments [47, 48], concluded that most of the
137Cs loss from the catchments was from
the eroded areas.
In 1975, Ritchie and McHenry [49] combined their data with those from the earlier studies of Rogowski and Tamura [39, 40, 41], Menzel [35], Frere and Roberts [36], and Graham [37], and found a significant exponential relationship between soil loss and radionuclide loss. This was encouraging since the data used to develop this empirical relationship came from a variety of soil and erosion conditions, used different radionuclides, and varied from fallout levels to high levels added as tracers. Their basic conclusion was that
137Cs would be a useful tool for measuring soil loss from the landscape.
While most of the early research in the 1970s was in the United States, in 1977 McCallan and Rose [50] used
137Cs and
210Pb to estimate erosion in a basin in Australia. The same year, Wise [51]
published a review paper in England on the use
137Cs and
210Pb to measure denudation rates.
Simultaneously, McHenry and Ritchie [52] found that
137Cs distribution in an agricultural field could also be used to show that most of the soil particles were being redeposited within the field rather than being lost from it. This opened a new area of interest in erosion, using the distribution of
137Cs to determine spatial patterns of erosion within a field and areas of net loss and of net gain (deposition) within a landscape element.
In the 1980s, four major centers of research on the use of
137Cs to study erosion were active.
McHenry and Ritchie [12] continued their activities in the United States. In Australia, Campbell [53], Elliott [54], Loughran [55], and others used the low levels of
137Cs in the southern hemisphere to measure erosion and sediment deposition. A group lead by de Jong [56] and his students [57,58] used
137
Cs extensively to study erosion on the Canadian prairie. In England, Walling [59, 60, 61] developed a center at Exeter University for using
l37Cs to quantify changes in landscape geomorphology. These centers are still active in their research efforts and continue to provide new methods to use
137Cs to quantify soil redistribution across the landscape.
4. EQUATIONS/MODELS
Empirical equations have been developed to explain the relationship between
137Cs and soil loss. The studies vary from simultaneous measurement of losses of soil and of
137Cs (radionuclide) from erosion plots, to correlation between soil loss from the plots and the reduction of
137Cs in these plots, to correlation between estimates of soil loss from fields and the reduction of
137Cs in the soils of these fields. Although important in showing a relationship between soil and
137Cs loss, these studies have many limitations. The general form of these equations is Y = aX
B. Such empirical equations are affected by climate, soils, time since fallout, time period of development, and other landscape and environmental factors. Similar concerns for the application and misuse of the empirical-based USLE have been expressed [8]. Empirical equations are applicable only to the data domain used in their development. Using this approach to estimate soil erosion will require the development of an empirical equation (calibration curve) for each site or at best, for each region. Empirical equations are dependent on the time since fallout and time of fallout. While they may help explain and better define the role of different factors that affect the relationship between soil loss and radionuclide loss, the many limitations to their application reduce their usefulness for estimating soil loss on a large scale.
A second approach for using
137Cs to study erosion is to assume that the loss of
137Cs at a site
is proportional to the loss of soil. The simplest form of this approach is to equate soil loss to
137Cs
loss (Y = X), where Y is soil loss in weight per area per time and X is
137Cs loss in percent or
weight. However, the X term is usually modified by depth distribution of
l37Cs, density of soil, decay
corrections, and other coefficients and modifiers [62]. Kachanoski [63, 64] provided an empirical
verification of this approach by measuring
137Cs concentrations in erosion plots at two different times and comparing the results of measured soil loss with measured
137Cs loss. A major assumption with this model is that
137Cs is instantaneously uniformly distributed in the soil profile. Since
137Cs is deposited on the surface and strongly adsorbed, it requires mechanical mixing (plowing) to achieve uniform distribution. Thus, during times of fallout, these conditions are seldom met leading to excess removal of
137Cs with surface erosion, causing overestimation of soil loss. Since fallout and erosion are both related to rainfall, there are concerns about erosion rates during times of heavy fallout.
During these times, greater erosion rates would remove proportionately more
137Cs from the surface layer of soil, leading to overestimation of long-term erosion rates. This problem is magnified if erosion rates are higher during maximum fallout, removing a disproportionate amount of the fallout.
Usually the proportional method overestimates erosion rates for periods of heavy fallout. The proportional method will probably reflect actual erosion rates for a cultivated area that was undisturbed during the fallout period.
Another approach is theoretical models/mass accounting. Walling and Quine [61] have defined the theoretical models as "the aggregate effect of all redistribution processes operating over the period since the initiation of atmospheric fallout to establish site-specific calibration relationships." Getting all the data needed to run a "process" based model may be a concern in some regions. By determining the factors that influence
137Cs movement, a
137Cs balance for a catchment can be developed and the spatial pattern and loss of
I37Cs from the catchment or field can be calculated. Such studies thus allow an understanding of erosion and deposition patterns of soil particles, and have been used [60, 65, 66]
to study erosion and deposition patterns on the landscape. These techniques give qualitative and quantitative information on erosion patterns. As with the other techniques, the balance approach requires a determination of the baseline level of
137Cs for the study area. Several studies have cautioned against using fallout measurements to estimate total
I37Cs loads in soils of a watershed.
Measurements of actual
137Cs should be made at a noneroding site in the catchment. By comparing
137
Cs measurement at a study site with the baseline level, one can determine whether erosion (less
I37
Cs present than at the baseline site) or deposition (more
137Cs present than at the baseline site) has occurred.
While there are limitations to the use of fallout
137Cs to estimate erosion, they are no greater than those associated with other techniques. Campbell et al. [67] suggest that the errors of the
137Cs technique may be less than current techniques used by soil scientists and geomorphologists to study erosion. If we understand the limitations and design studies to reduce the effects of these limitations, then measuring the concentration of
137Cs across the landscape can provide accurate and valuable information on erosion.
5. CONCLUSIONS
Thirty years of research have shown that the distribution of fallout
137Cs on the landscape can be used to measure soil loss. Measurements of spatial patterns of
137Cs can provide unique insights into soil movement and redeposition within a landscape element that other methods cannot provide.
The
137Cs method is often the only way to get actual measurements of soil loss and redeposition. As
such, research should continue the development of the technique to better understand the changing
landscape. Applications should also be encouraged in areas of the world where resources are limited
for developing long-term erosion monitoring programs.
REFERENCES
[I] BROWN, L.R., WOLF, E.G., Soil erosion: Quiet crisis in the world economy, Worldwatch Paper 60 (1984) 51 pp.
[2] PIMENTEL, D., et al., World agriculture and soil erosion, Biosci. 37 (1987) 277-283.
[3] WALLING, D.E., "The struggle against water erosion and a perspective on recent research", Water Erosion, (K.
Ivanov, Pechinov, D., Eds.), UNESCO Technical Document in Hydrology SC-89/WS-57 (1989) 39-60.
[4] CLARK, II, E.H., The off-site cost of soil erosion, J. Soil Water Conserv. 40 (1985) 19-22.
[5] COLACICCO, D., et al., Economic damage from soil erosion, J. Soil Water Conserv. 44 (1989) 35-39.
[6] LAFLEN, J.M., et al., WEPP A new generation of erosion prediction technology, J. Soil Water Conserv. 46 (1991) 34-38.
[7] W1SCHMEIER, W.H., SMITH, D.D., Predicting rainfall-erosion losses for cropland east of the Rocky Mountains, Agr. Handbook No. 262 USDA, Washington, DC (1965).
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silt loam, Health Phys 11 (1965) 1333-1340
[40] ROGOWSKI, A S , TAMURA, T , Environmental mobility of cesmm-137, Rad Bot 10 (1970) 35-45 [41] ROGOWSKI, A S , TAMURA, T , Erosional behavior of cesium-137, Health Phys 18 (1970) 467-477 [42] DAHLMAN, R C , AUERBACH S I , Preliminary estimation of erosion and radiocesium redistribution in a
fescue meadow ORNL-TM-2343 (1968), Oak Ridge National Laboratory, Oak Ridge, Tennessee
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[44] RITCHIE, J C , et al , Distribution of cesium-137 in the upper 4 inches of soil in relation to vegetation type ASB Bulletin 17 (1970) 60
[45] RITCHIE, J C , et al , Estimating soil erosion from the redistribution of Cs-137 fallout Agronomy Abstracts 1973 Annual Meeting (1973) 129
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Sons, London (1990) 33-53.
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NEW PERSPECTIVES ON THE SOIL EROSION-SOIL QUALITY RELATIONSHIP
D.J. PENNOCK """"""'XA9847765 University of Saskatchewan,
Saskatoon, Saskatchewan, Canada
Abstract
NEW PERSPECTIVES ON THE SOIL EROSION-SOIL QUALITY RELATIONSHIP
The redistribution of soil has a profound impact on its quality (defined as its ability to function within its ecosystem and within adjacent ecosystems) and ultimately on its productivity for crop growth. The application of "7Cs -redistribution techniques to the study of erosion has yielded major new insights into the soil erosion-soil quality relationship. In highly mechanized agricultural systems, tillage erosion can be the dominant cause of soil redistribution; in other agroecosystems, wind and water erosion dominate. Each causal factor results in characteristic landscape-scale patterns of redistribution. In landscapes dominated by tillage redistribution, highest losses occur in shoulder positions (those with convex downslope curvatures); in water-erosion-dominated landscapes, highest losses occur where slope gradient and length are at a maximum.
Major impacts occur through the loss of organically-enriched surface material and through the incorporation of possibly yield-limiting subsoils into the rooting zone of the soil column. The potential impact of surface soil losses and concomitant subsoil incorporation on productivity may be assessed by examining the pedological nature of the affected soils and their position in the landscape. The development of sound conservation policies requires that the soil erosion-quality relationship be rigorously examined in the full range of pedogenic environments, and future applications of the 137Cs technique hold considerable promise for providing this comprehensive global database.
1. INTRODUCTION
The connection between soil redistribution and soil quality is implicit in many definitions of soil quality. For example, Larson and Pierce [1] define quality as the capacity of a soil to function, both within its ecosystem boundaries (e.g. soil map unit boundaries) and with the environment external to that ecosystem (particularly relative to air and water quality). The basic functions of a soil within the ecosystem are to sustain biological productivity, maintain environmental quality, and to promote plant and animal health [2].
Clearly, soil erosion has a range of ecosystem effects: on the soil itself at the point where erosion occurs; on the landscape where redistribution of soil occurs; and on adjacent ecosystems to which the soil is exported (primarily downstream aquatic ecosystems). Although all three scales of study are important, this review concentrates on the impact of erosion at the pedon and landscape scales; the primary focus is on the new insights into the soil erosion-soil quality relationship that have been gained by application of the
137Cs redistribution technique.
Redistribution alters the chemical, biological, and physical composition of soil at each point
hi the landscape where it occurs (the pedon scale). These changes in composition may affect the ability
of the soil to perform the ecosystem functions outlined above. The literature in soil science and related
disciplines abounds in specific examples of the impact of redistribution on soil quality, yet few
generalizations have emerged from this voluminous body of work. In most cases, at the point where
loss occurs, there will be a decrease in the ability of the soil to function. The impact of soil deposition
on function is even less clear: positive and negative impacts on soil quality have been observed. The
link between these redistribution-related changes in soil quality and in crop productivity is also elusive.
Commonly, conclusions are based on what should happen to crop yields given the severity of soil- quality changes, rather than on what has been observed to occur.
The redistribution of soil materials also has a profound influence on the spatial pattern of soil quality indicators within the ecosystem boundary (the landscape scale). Redistribution can increase the range of variability within a given landscape unit; however, redistribution also imposes, or reinforces, a distinctive landform-soil property relationship that can be used to stratify landscapes into meaningful response units or experimental units. Hence, although redistribution may increase the overall range of variability, it can also create or exaggerate an overall spatial order within the ecosystem.
Soil transport by redistribution beyond the boundaries of the source ecosystem to adjacent ecosystems (the regional scale) is the final scale of relevance in examining impact on ecosystem function. The sediment itself and the chemical and biochemical components sorbed to it can be significant contributors of soil-derived pollutants to aquatic ecosystems. The effect of soil erosion on air quality, through deflation and transport by wind, can be of importance in specific regions (i.e. the dusts of north-central Africa), but tends to be a short-lived phenomenon elsewhere in the world.
A rigorous evaluation of the causes and impacts of soil erosion at all the scales of relevance is critical for the development of scientifically-sound land-use policies. The need for reliable data upon which to base these planning strategies is starkly illustrated in two recent articles. Pimentel et al. [3]
argue that one-third of the world's arable land has been lost to erosion in the past 40 years; the cost of erosion losses in the U.S.A. alone totals $44 billion per year, and they suggest that these losses could be reduced to a sustainable levels with total expenditures of $ 8.4 billion per year. In his response to their study, Crosson [4] suggests that their estimate of arable land lost "rests on such thin underpinnings that it cannot be taken seriously" (p. 461). His estimates of the annual cost of erosion- induced productivity losses in the U.S.A. are in the range of $500 to $600 million (1986 dollars).
Clearly we cannot expect policy-makers to develop sound conservation policies when we are unable to provide a more authoritative data set upon which to base decisions.
2. CHANGING CONCEPTS OF SOIL REDISTRIBUTION
The great contribution of soil-geomorphologists such as R. Rune and R. Daniels was to establish that soil redistribution is a natural process that has driven landscape evolution throughout time. The focus of this review is on the increases in soil loss and gain associated with human activity, or, as it is more commonly known, accelerated soil erosion.
2.1. Causes of soil erosion
Traditionally, reviews of the causes of "erosion" have focused almost exclusively on wind and water processes as the major determinants of soil redistribution. The physical processes of wind and water erosion, their relationship to soil and landform properties, the domains in which they operate, and the relative importance of each process in those domains have been exhaustively studied, and many fine reviews exist of this research [e.g. 5].
Recent research on tillage operations [6, 7, 8, 9] has, however, challenged the view that only
wind and water erosion need to examined as the dominant causes of soil redistribution in all
landscapes. Working at research sites in agricultural landscapes of Canada and Europe, these authors
used a variety of techniques (natural and enriched
137Cs redistribution, displacement of simulated
clods, simulation modelling) to examine the relative importance of water and tillage erosion. Their
results strongly support the idea that tillage redistribution is a major cause of soil movement in agricultural landscapes.
Cultivation can displace soil upslope or downslope, depending on the direction of the operation; however, because downslope displacement is greater than upslope displacement, a net downslope displacement occurs. The rates of loss attributed to these practices (discussed below) can be high; however, the magnitude of the loss depends on the type and sequence of the tillage operation, as well as on the type of implement and the speed of operation [9].
Clearly, the impact of tillage redistribution is of greatest importance in highly mechanized agricultural systems - in other systems such as "slash and burn" described by Garcia-Oliva et al. [10], other causes of redistribution predominate. The recognition of the relative importance of a given erosion process (water, wind, or tillage) in a specific region is critical for explaining the landscape- scale spatial patterns of loss and gain, as well as for evaluating possible extra-ecosystem impact of soil redistribution.
2.2. Rates of soil redistribution as determined using
I37Cs techniques
Observations of rates of soil redistribution are available from a variety of sources. In many cases, they have been made on small research plots to generate data for the development and calibration of predictive erosion models such as the Universal Soil Loss Equation (USLE) or the forthcoming Water Erosion Prediction Project (WEPP). Traditionally, however, these plots were designed to examine only the soil-loss part of the redistribution continuum, hence they are limited in their usefulness when examining redistribution as a whole. Far too often the measured rates of soil loss from these research plots are uncritically extended to the landscape as a whole, without an evaluation of potential redistribution within the source landscape [11].
TABLE I. EXAMPLES OF RECENT VALUES FOR SOIL LOSS Location Field Type Rate of soil loss
in eroded postions of the field
(Mgha
1yf
1)
Reference
Mexico
U.K., Belgium Ontario, Canada Saskatchewan, Canada
Pasture and undisturbed forest Cultivated Cultivated Cultivated
13
10 to 20 68 to 82 20 to 30
[10]
[8]
[9]
[15, 16]
The radionuclide
l37Cs was deposited on the soil chiefly as nuclear-bomb fallout in the 1950s and 1960s; peak deposition occurred in 1963 in the northern hemisphere, and some areas received more from the Chernobyl disaster in 1986. Upon deposition,
137Cs is tightly bound by colloidal material at the soil surface. Redistribution rates of these mineral-
I37Cs complexes is highly correlated with observed rates of soil redistribution [12]. The increasing use of
137Cs is providing valuable sets of observations on redistribution rates within actual agricultural landscapes [13].
The most common research design used in
137Cs studies has been to sample soils along transects or grids. After calculation of the change in
137Cs concentration through time due to redistribution, usually by comparing the levels in the agricultural field to a nearby uneroded or reference site, a value for soil loss or gain at the specific sampling point can be determined. At each study site, a certain proportion of samples will show loss and some may show gain; the overall balance indicates the net export, or, less commonly, import, of soil.
137
Cs-derived rates of soil redistribution differ widely among landscapes, regions, and agricultural systems; however, as a generalization for mid-latitude regions, those portions of the landscape where soil loss occurs usually show average rates of 10 to 60 Mg ha"
1(Table I); typical average losses are around 151 ha"
1yr'
1(although loss at specific points may be an order of magnitude greater). Note, however, that this average figure does not represent soil export from the study site;
in many cases, much of the eroded soil is deposited within the source landscape, especially those dominated by tillage erosion.
Deposition rates are considerably more variable than erosion rates. Typically, deposition is concentrated in a small proportion of sampling points within a landscape [14, 15], but rates at specific points may be high.
For large areas of the tropics, including most of Africa and South America,
137Cs results are not available. Lal [5] used existing figures for sediment transport in African river systems to estimate net erosion losses in upstream areas. Most of the African continent shows net erosion losses of < 10 (arid areas) to 50 Mg ha"
1yr"
1. The highest losses occur in the Maghreb region of NW Africa, with losses of >75 Mg ha"
1yr"
1.
The rates of redistribution have greater relevance for soil quality if we convert them into depth of soil lost or gained. Using the bulk densities of a surface prairie soil presented in Pennock et al.
[16], a 10 Mg ha"
1yr"
1soil loss from a soil with a bulk density of 1.07 (native prairie) translates to a loss of 0.093 cm yr"
1; the same rate of loss from a soil with a bulk density of 1.42 (a site cultivated for 80 years) corresponds to 0.070 cm yr"
1. The loss per year is not dramatic; however, over a period of cultivation, it may be substantial.
The sites discussed above are regionally typical agricultural landscapes, generally excluding representatives of high magnitude-low frequency (or catastrophic) erosion events. Sites affected by extreme events (e.g. high intensity rainfall, major wind erosion events) or locations within a field that have been selectively influenced by erosion processes (e.g. landscape positions with extensive gullying) are likely to show much higher rates of loss.
3. SOIL REDISTRIBUTION-SOIL QUALITY INTERACTIONS: THE PEDON SCALE
The impact of redistribution on specific points in the landscape (herein called the pedon scale)
must begin with an understanding of the place of that pedon in the overall landscape. The action and
interaction of soil processes that result in the properties observed in a given pedon do not occur at
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